1. Field of the Invention
The invention relates to a clock generation circuit, and more particularly to a clock generation circuit whose output clock signal can be adjusted to be within a frequency ppm range defined by a specification.
2. Description of the Related Art
FIG. 1 shows a conventional transmission device 1, which comprises a transmitter 10, a phase locked loop (PLL) circuit 11, and a clock generator 12. The clock generator 12 generates a reference clock signal CLKref, which is provided to the PLL circuit 11. The PLL circuit 11 generates a clock signal TXCLK to the transmitter 10 according to the reference clock signal CLKref, and the transmitter 10 transmits data with the clock signal TXCLK. Since the clock signal TXCLK is generated according to the reference clock signal CLKref, the frequency ppm range of the clock signal TXCLK is dominated by that of the reference clock signal CLKref, such as the specification of SATA (Serial Advanced Technology Attachment) restricts output clock frequency within +/−350 ppm without using a spread spectrum clock (SSC) component. The specification of a high-speed link usually defines a ppm range for frequency of transmitted data. If the frequency of the reference clock signal CLKref does not conform to the defined frequency ppm range of the specification, the frequency of the clock TXCLK is hard to conform to the defined frequency ppm range, so that the transmitted data eventually does not meet the specification.
In general, the clock generator 12 is implemented by an on-board clock source, such as a crystal or a resonator. A crystal has a concentrated frequency ppm range; however, compared with a resonator, the crystal has a higher cost. In the other hand, it is difficult controlled the frequency of the resonator, which is a cheaper choice of the clock generator 12, to conform to a frequency ppm range defined by a required specification. Thus, if a resonator is used for implementation for lower costs, it is likely that the frequency of the reference clock signal CLKref will possible not conform to the defined ppm range.
Thus, it is desired to provide a clock generation device for a transmitter, which uses a low-cost reference clock generator and generates a clock signal whose frequency conforms to the frequency ppm range defined by a specification, so that the output data of the transmitter meet the specification.